There has recently been an increasing tendency toward producing lightweight and compact optical elements such as lenses by employing aspherical lenses made of a high refractive index and high dispersion optical glass and thereby reducing the number of lenses. An attempt for obtaining a aspherical surface by using the conventional grinding and polishing processes, however, requires many costly and complex working processes. Hence there has been developed a method for forming a preform made of a glass gob or glass block directly into lenses by employing a super-precision processed mold. Since the lenses made by this method need not be subjected to grinding and polishing, it has become possible to produce lenses at a lower cost and within a shorter period for delivery. A great deal of research and development have been made on this lens forming method which is called “glass molding” and aspherical lenses for optical instruments made by glass molding have been increasing year after year.
As a glass used for glass molding is sought a glass which is softened at a lower temperature by reason of the heat resisting property of the mold used for glass molding. Optical constants of a lens sought for this purpose are also diversified. For example, as described in Japanese Patent Application Laid-open Publication No. Hei 2-148010 and No. Hei 6-160712, for realizing an optical design which meets a compact and high specification, there is an increasing demand for using high refractive index and high dispersion aspherical lenses. Particularly, optical constants of a refractive index (nd) within a range from 1.825 to 1.870 and an Abbe number (ν d) within a range from 22 to less than 27 belong to ranges of the highest refractive index and the highest dispersion for an optical glass sought in this filed of art and, if it becomes possible to manufacture an optical glass having optical constants within these ranges, lenses produced from this optical glass will be able to realize correction of chromatic aberration in a compact optical system at a low cost.
There is, therefore, a strong demand for development of an optical glass suitable for mold pressing having optical constants within these ranges which is very useful for an optical design. However, prior art optical glasses for mold pressing having optical constants within these ranges have poor chemical durability and insufficient resistance to thermal shock and, therefore, chipping and breaking of a glass gob take place frequently during forming of a preform and pressing. Thus, there has been no practical optical glass suitable for mold pressing.
There have been many prior art optical glasses which have optical constants in the vicinity of the optical constants which are desired in the present invention. For example, Japanese Patent Application Laid-open Publication No. 2001-058845 and No. 2002-173336 disclose optical glasses having a phosphate. These glasses, however, do not have sufficient chemical durability and tend to be fused to the press mold. Besides, these glasses have such a high mean coefficient of linear thermal expansion (α) that the glasses tend to be damaged during quick cooling or quick heating applied before or after pressing and hence they are not suitable for press forming.
Japanese Patent Application Laid-open Publication No. Sho 58-217451 discloses an optical glass for mold pressing containing a large amount of P or Pb. A glass containing a large amount of P or Pb, however, is highly reactive to a mold in the pressing temperature range and therefore tends to deteriorate the mold and hence it is not suitable for an optical glass for press forming.
Japanese Patent Application Laid-open Publication No. Sho 48-034913 discloses a K2O(Na2O)—SiO2—TiO2—Nb2O5 optical glass. This glass, however, is not suitable for press forming because it has transformation temperature (Tg) which is too high for an optical glass for press forming which is looked after today and, moreover, it has a large mean coefficient of linear thermal expansion (α) and, therefore, it tends to be damaged during quick cooling or quick heating applied before or after pressing.
Japanese Patent Application Laid-open Publication No. Hei 1-148726 and Japanese Patent Application Laid-open Publication No. Hei 5-051233 disclose glasses which are made of a Na2O—SiO2— TiO2—Nb2O5 glass added with Ge or Cs. However, these glasses are costly because they require expensive raw materials.
Japanese Patent Application Laid-open Publication No. Sho 52-45612 discloses a R2O—RO—SiO2—Nb2O5 optical glass. This optical glass, however, has a low refractive index (nd) and a large Abbe number (ν d) and therefore desired optical constants of high refractive index and high dispersion cannot be achieved by this optical glass. For this reason, this optical glass is not suitable for an optical glass for mold pressing which is required today.
Japanese Patent Application Laid-open Publication No. 2002-87841 discloses a SiO2—TiO2—Nb2O5—Na2O glass as a precision press forming material. Since this glass fails to satisfy either of refractive index (nd), Abbe number (ν d) and mean coefficient of linear thermal expansion (α) which are required today, it is not suitable for a highly refractive optical lens for press forming.
Japanese Patent Application Laid-open Publication No. 2000-016830 discloses an optical glass having a low transformation temperature (Tg). Since, however, this optical glass has a large Abbe number (ν d), i.e., low dispersion) or a low refractive index (nd), the optical constants of this optical glass are outside of the optical constants required for an optical glass for mold pressing which is the object of the present invention.
Japanese Patent Application Laid-open Publication No. 2000-344542, No. Sho 61-168551, No. Sho 54-161619, No. Sho 54-161620, No. Sho 49-087716, and No. Sho 58-125636 disclose highly refractive glasses for spectacles which have an mean coefficient of linear thermal expansion (α) below 100 {10−7° C.−1}. These glasses, however, have a larger Abbe number (ν d) or a lower refractive index (nd) than that of the present invention and, therefore, the optical constants of these glasses are outside of those of an optical glass for mold pressing which is the object of the present invention.
It is an object of the present invention to provide an optical glass for mold pressing which has the above described desired optical constants, a low transformation temperature (Tg) and high resistance to thermal shock.
Thermal stress (α) can generally be predicted by the following formulaσ=λ·E·α·ΔT/(1−ν)  (A)
where λ repesents a constant relating to the shape and heat transfer speed, E Young's modulus, α mean coefficient of linear thermal expansion, ΔT temperature difference and ν Possion's ratio.
For reducing thermal stress σ which is represented by the formula (A), it is conceivable, for example, to reduce ΔT. For avoiding abrupt heating and cooling in press forming, it is conceivable to provide a preliminary furnace in a stage before or after the forming process thereby to mitigate abrupt temperature change (ΔT) in the process of rising and falling of temperature. Since, however, this method causes a preform to stay for a long period of time in the preliminary furnace and the forming process which constitute a high temperature condition for a glass, there is high possibility that fine crystals grow in the glass, i.e., devitrificatin takes place. Besides, the provision of one or more preliminary furnaces makes the facility more complicated and prolongs cycle time required for producing a single product with resulting increase in the production cost. Accordingly, there is a limit in the method of reducing the temperature change ΔT in the press forming.
Among the parameters of the above formula (A), values which depend largely on the glass composition are Young's modulus (E), mean coefficient of linear thermal expansion (α) and Poisson's ratio (ν). It is, therefore, important for an optical glass for mold pressing having high resistance to thermal shock to obtain a material which has small values of Young's modulus (E) and mean coefficient of linear thermal expansion (α).